In the in line gun color tube three electron beams are emitted and accelerated towards a screen coated in groups of red green and blue phosphors. The phosphor groups are deposited on the screen in alignment with a perforated metal mask located close to the screen.
Since the electron guns are in the same plane, with the center gun on the tube axis, and providing they are not subjected to any external deflecting field, the electron beams will coincide at the screen center, creating static convergence.
However, the electron beams are intercepted by the perforated metal mask, such that each gun's electron beam can hit a single colored phosphor and as a consequence create good color purity.
There are inherent problems when the electron beam is deflected in color picture tubes which make make it necessary to adjust both electronic beam convergence and color purity.
U.S. Pat. Nos. 4,162,470 and 4,641,062 indicated that these adjustments can be made by inducing permanent magnetization inside a strip wrapped around the tube neck. The strip can be composed, for example, of a powdered magnetizable material dispersed in a plastic binder material. This plastic binder is attached to an adhesive film surface which also provides attachment to the tube neck.
The prior art states that the strip is magnetized in place to control static convergence and purity. This magnetization is performed by a device placed in contact with the strip. However, this method has two disadvantages:
when the strip is wrapped around the neck, the last section of magnetizable strip is prevented from making contact with the neck by the adhesive tape, which, to complete encirclement overlaps the start of the magnetic strip and results in excessive thickness.
the tolerances on tube neck diameter require that the length of the magnetic strip is shorter than the upper limit of neck circumference, which avoids the unwanted overlapping but results in a gap with no magnetizable material.
The magnetization device usually consists of a ring of coils introduced coaxially to the tube neck and located in contact with the strip to be magnetized. Excess thickness can prevent location or may cause tearing of the strip during the positioning of the magnetization device. When positioned coaxially, any excess thickness may space the magnetization device from the strip and prevent good contact with the periphery. This variation in contact will result in improper magnetization of the strip. The goal of the present invention is to describe a static convergence and/or purity device which can be substituted for the device described in current technology without the previously mentioned disadvantages.